Low noise multilayer transformer

ABSTRACT

A magnetic structure in a power converter includes at least two multilayer boards, such as a primary board containing the primary windings and some auxiliary windings, and a secondary board containing the secondary windings and some auxiliary windings. The primary and secondary boards are on top of each other. On the layer on the primary bard adjacent to the secondary board, is a dual function shield to reduce the total common mode noise in the converter towards zero. The controlled dual function shield can be placed on the secondary board on the layer adjacent to the primary board, and in some embodiments can be placed on both primary and secondary board on the layers adjacent to the other board. The embodiments herein offer a very good solution for cost reduction of the planar transformers and offers an avenue for total elimination of the common mode noise in a power converter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.63/112,564, filed Nov. 11, 2020, which is hereby incorporated byreference.

FIELD

The present invention relates generally to electronic power conversion,and more particularly to magnetic transformers.

BACKGROUND

In the quest for high density power converters, there is a focus is onthe efficiency and also and also in the miniaturization of the magneticelements and on the reduction of the number of components.

In the recent years there is a trend for the power adapters to decreasethe size, fueled by the fact that the electronic devices powered by theAC-DC adapters such as laptops, mobile phones have decreasedsignificantly in size.

One key element in a power converter is the magnetic transformer. Themagnetic transformer occupies a significant volume in the poweradapters, ranging from 20% to 40% or even more. The embodiments of thispatent offers several solutions in increasing the power density of themagnetic transformers in power conversion application with very highdensity while minimizing the power dissipation. In the same time in therecent years adapters with power delivery have been introduced whereinthe output voltage range varies from 3.3V to 20V which is a range of6:1. The input voltage range varies from 90 Vac to 265 Vac, that is arange of 3:1. With such a large input and output range the most suitabletopology for the AC-DC adapters is flyback topology. Unlike the forwardtopology, wherein the magnetic transformer is used to transfer powerfrom primary to secondary via the isolation boundaries, the flybacktransformer provide two functions. The first function is to transferpower from primary to secondary during the off time of the main switchwhile storing energy in the transformer during the on time of the mainswitch. In forward topology, the power transformer transfers the energyform primary to secondary during the time wherein the primary switchersare on and there is another magnetic element, referred as the outputchoke, which function is to store the energy during the time the primaryswitchers are on and further to transfer that energy to the outputtowards the load when the primary switchers are off.

In Flyback topology both functions are produced by only one magneticelement, referred in this application as Flyback transformer.

Presently, most laptops, tablets require for operation a power levelranging from 30 W to 65 W. Due to the significant technologicaladvancement in portable computing devices, the size of laptops andtablets has been significantly reduced, and yet the AC-DC adapters usedto power such devices remain quite large (for example, dimensions of atypical adaptor for a small tablet device are about 3.3″ by 1.8″ by 1.3″or so). This has created pressures for size reduction of AC-DC adapters.To reduce the size of the adapters while maintaining theconvection-based cooling methodology used today requires somesignificant improvement in efficiency of the adapters as well as adecrease of size of the magnetic and capacitive storage elements.

Besides the power transformer, another section of the AC-DC adapterwhich occupies a large volume is the input EMI filter. The EMI filteroccupies approximately 10%-20% of the volume of an AC-DC adapter.

This patent will present several embodiments which will integrate someof the common mode EMI filter functions in the transformer itself.

SUMMARY

In one embodiment, and without limitation, a system for reducingcommon-mode noise includes components of a switch mode power supplyincluding primary and secondary sides, primary and secondary sidegrounds, an input voltage source, a primary switch, a transformer, acore, and a power output, wherein the primary side and the secondaryside have primary and secondary quiet terminations, respectively,wherein voltage does not change with respect to the primary side groundand with respect to the secondary side ground. A magnetic elementcontains a magnetic core having at least two magnetic legs. The systemincludes a primary multilayer board which embeds primary windings andfirst auxiliary windings which report to the primary windings, whereinthe primary windings and the first auxiliary windings surround themagnetic legs. The system also includes a secondary multilayer boardwhich embeds secondary windings and second auxiliary windings whichreport to the secondary windings, wherein the secondary windings andsecond auxiliary windings surround the magnetic legs. The magnetic corepenetrates both the primary multilayer board and the secondarymultilayer board. The primary and secondary multilayer boards are placedon top of each other. A layer of the primary multilayer board which isadjacent to the secondary multilayer board includes a controlled dualfunction shield which is connected to the primary quiet termination.

The above provides the reader with a very brief summary of someembodiments described below. Simplifications and omissions are made, andthe summary is not intended to limit or define in any way thedisclosure. Rather, this brief summary merely introduces the reader tosome aspects of some embodiments in preparation for the detaileddescription that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings:

FIG. 1 presents a schematic of a flyback convertor;

FIG. 2 depicts one of the embodiments wherein a transformer is built bytwo multilayer boards, one primary board and a secondary board and amagnetic core;

FIG. 3 represents the structure of the primary and secondary board.

FIG. 4 presents the secondary circuit in a flyback topology in high sideimplementation.

FIG. 5 presents the secondary circuit in a flyback topology in low sideimplementation.

FIG. 6A presents one of the key embodiments, “Controlled dual functionshield”;

FIG. 6B presents one of the key embodiments, “Controlled dual functionshield” with an example of the location of the total cutout;

FIG. 6C presents one of the key embodiments, “Controlled dual functionshield” with another example of the location of the total cutout;

FIG. 6D presents one of the key embodiments, “Controlled dual functionshield” with the total location diametral opposed to the “quiettermination” location;

FIG. 6E presents one of the key embodiments, “Controlled dual functionshield” with more than a turn design for the shield;

FIG. 6F presents one of the key embodiments, “Controlled dual functionshield” with more than a turn design for the shield but oppositepolarity to the example from FIG. 6E;

FIG. 7 illustrates one of the embodiments, wherein the secondary uses“low side” SR implementation and the secondary board does not have ashield;

FIG. 8 illustrates one of the embodiments, wherein the secondary uses“high side” SR implementation and the secondary board does not have ashield;

FIG. 9 illustrates one of the embodiments, wherein the secondary uses“high side” SR implementation and the secondary board does have ashield;

FIG. 10 illustrates one of the embodiments, wherein the secondary uses“high side” SR implementation and the secondary board does have ashield;

FIG. 11A illustrates one of the embodiments, wherein the secondary boardhas some components on it, and it is placed on top of primary board;

FIG. 11B illustrates one of the embodiments, wherein there are twosecondary boards placed on top and bottom of the primary board and saidsecondary boards have some components on it;

FIG. 12 illustrates one of the embodiments, wherein there areinterconnection pins to connect the primary and secondary board to themother board wherein the primary board is on top of the secondary board;

FIG. 13 illustrates one of the embodiments, wherein there areinterconnection pins to connect the primary and secondary board to themother board, wherein the secondary board is on top of the primaryboard;

FIG. 14 illustrates one of the embodiments, wherein primary board andsecondary board are connected by the use of plated half hole cutinterconnection type;

FIG. 15 illustrates one of the embodiments, wherein the magnetic corehas multiple posts which penetrates primary and secondary boards;

FIG. 16 illustrates one of the embodiments, wherein the primary andsecondary boards are applied to a two-transistor forward topology; and

FIG. 17 illustrates one of the embodiments, wherein the secondary boardis placed in between primary boards in flyback topology.

DETAILED DESCRIPTION

Reference now is made to the drawings, in which the same referencecharacters are used throughout the different figures to designate thesame elements. Briefly, the embodiments presented herein are preferredexemplary embodiments and are not intended to limit the scope,applicability, or configuration of all possible embodiments, but ratherto provide an enabling description for all possible embodiments withinthe scope and spirit of the specification. Description of thesepreferred embodiments is generally made with the use of verbs such as“is” and “are” rather than “may,” “could,” “includes,” “comprises,” andthe like, because the description is made with reference to the drawingspresented. One having ordinary skill in the art will understand thatchanges may be made in the structure, arrangement, number, and functionof elements and features without departing from the scope and spirit ofthe specification. Further, the description may omit certain informationwhich is readily known to one having ordinary skill in the art toprevent crowding the description with detail which is not necessary forenablement. Indeed, the diction used herein is meant to be readable andinformational rather than to delineate and limit the specification;therefore, the scope and spirit of the specification should not belimited by the following description and its language choices.

In U.S. Pat. No. 5,990,776 entitled “Low noise full integratedmultilayers magnetic for power converters” Jitaru is presented atechnology wherein the winding of the magnetic elements such is thetransformer are embedded in the multiplayer PCB, multilayer PCB which isalso the mechanical support for the power components and signalcomponents. A planar transformer has a larger footprint than aconventional transformer and that will reduce the space for the rest ofthe components on a given package. The main embodiment of Jitaru patentwas that all the winding of the planar transformer to be sandwichbetween at least two layers, a top layer and a bottom layer, layerswhich are used as support for components especially the key powercomponents, such as the synchronous rectifiers, which have to be placedclose to the transformer secondary winding in order to minimize thestray inductance. In Jitaru patent the footprint of the transformer isreduced to the footprint of the magnetic core.

Other embodiments of Jitaru patent are related to the common mode noisereduction by placing the windings in the planar transformer in a suchway to minimize the displacement current through the parasiticcapacitance in between the windings especially through the parasiticcapacitance between primary and secondary windings.

One disadvantage of the embodiments in the U.S. Pat. No. 5,990,776 isthe fact that the primary windings and the secondary windings are placedon the same multilayer PCB and the parasitic capacitance in betweenprimary winding and secondary is high increasing the common mode noise.Jitaru presents several solutions to decrease common mode noise by theproper positioning of the windings per layers. The prepreg material usedto merge the multilayers of copper foil does have a permeability between5 to 7 or even higher. That will also increase the parasitic capacitancebetween two adjacent layers of copper. Another drawback of this conceptis the fact that by placing both, primary and secondary on the samemultilayer PCB increases the number of layers which increases the costof the multilayer PCB. In addition to that, in many applications thecopper thickness in some of the secondary windings or even the primarywindings need to be higher than other layers and that will increase thecomplexity and the cost of the multilayer PCB transformer.

Though the embodiments of this invention apply to the transformers usedin any topology, the description of the embodiments is focused primarilyon the flyback topology, but these embodiments can be used in anytopology.

In power conversion applications the coupling through the parasiticcapacitance between components in the primary side and the secondaryside, wherein the voltage on said components varies with a given dV/dtamplitude, displacement current is produced, referred in this patent as“non transformer common mode noise”. The “non transformer common modenoise” is adding to the Common Mode EMI produced by displacement currentthrough the transformer. In transformers, the parasitic capacitancesbetween primary to secondary windings leads to displacement currents dueto the voltage variation (dV/dt) in the windings. Some shieldingtechniques can be used in transformers to minimize the displacementcurrent between primary and secondary windings. However, even in theevent all the common mode noise created in the transformer issignificantly reduced, the “non transformer common mode noise” stillremains.

In such cases the goal is to create a “controlled displacement current”in the transformer which has the same amplitude and of opposite polarityof the common mode current created by the other sources in theconverter. This technique is known in the field of power conversion asnoise cancelation technique.

First Embodiment

In FIG. 1 is presented a simplified schematic of a flyback topology.This basic structure of a flyback topology contains a primary switch130, a transformer 105, containing a primary winding 110, and asecondary winding 120. The primary winding 110, is connected with oneterminal to the dc input voltage V_(Bulk+), 100, and with the secondterminal to the drain of the primary switch 130. The secondary windingis connected with a terminal to a synchronous rectifier, 140, and thesecond terminal to an output capacitor Co, 170. The input section of theflyback is connected to the input GND, 150. The output section of theflyback is connected to the output GND, 160. Both windings, the primarywinding 110 and the secondary winding 120, are characterized by apolarity which is defined by the position of the dot, 115. When an acvoltage source is applied to one of the windings of a transformer withthe positive polarity of said voltage source at the dot, an ac voltagesynchronized with said voltage source will be induced in the rest of thewindings with the positive polarity at the dot.

In the first embodiment the primary winding is embedded in onemultilayer PCB, 220 as depicted in FIG. 3. In FIG. 3 is presented thestructure of a multilayer board for primary windings, named the primaryboard, 220 and the structure of a multilayer board for secondarywindings, named the secondary board, 280. The inner layers 530, 540,550, contain the primary winding, as an example. The top layer, 500 isused as a primary shield which is electrically connected a “quiettermination” for example, the Vbulk, 100, or to input GND, 150, fromFIG. 1. There is another primary shield placed on the bottom layer, onthe other side of the primary multilayer PCB, on layer 510. A portion ofthe primary shield, 510 is metalized with the purpose in someapplications to make electrical contact with the magnetic core.

As depicted in FIG. 3, the secondary board 280, contains the secondarywinding in the layers 570 and 580 as an example. The layer 600 and layer590 can be used for the secondary shield. The top layer 240 and thebottom layer 250 will not be metalized in this example. The distance,610, between the layer 590 and the layer 240 and in between layers 600and 250 also shall have the distance 610, distance 610 which isnecessary to comply with the safety agency in many of the cases thatdistance is 0.4 mm or higher.

In FIG. 4 is depicted the configuration of the windings in the secondaryboard, 280. The secondary windings 120, is placed for example on thelayers 570 and 580. There can be two configurations for the secondary.The first configuration from FIG. 4 is referred in the power conversionfield as “high side” rectifier, wherein the source of the synchronizedrectifier, 140, is connected to the secondary winding, 120, at thenon-dot termination.

In FIG. 4, the dot termination of the secondary winding 120, isconnected to the secondary GND, 160. The shield 600 will be placed underthe secondary windings layers. The winding on the layer 580 is connectedto the secondary GND, 160 as depicted in FIG. 4. We introduce in thedescription of the embodiments of this patent the term of “quiettermination”. A quiet termination” in the primary is the primary GND,160 and the Vbulk+, 100. In the secondary a “quiet termination” is thesecondary GND, 160 and the Vo+”, 320. Another terminology which weintroduce in this patent is the term of “quiet winding”. A quiet windingis the winding which has a low voltage swing reported to the “quiettermination”, by comparison with the rest of the windings.

The layer 600 of the secondary board 280, which will be placed on top ofthe primary board, 220, is also named the bottom layer of the secondaryboard, this is because the layer 250, is not metalized. The layer 240,of the secondary board, 280, is named the top layer of the secondaryboard and in many application it is not metalized in order to complywith safety agencies.

The layer 500, of the primary board 220 is named the top layer of theprimary board, and the layer 510, is named the bottom board of theprimary board. As depicted in FIG. 2, the bottom layer of the secondaryboard, 250 is placed above the top layer, 500, of the primary board. Thetop layer of the primary board is allocated for the shield. The purposeof the shield is to prevent the displacement current induced by theprimary winding, via the parasitic capacitance between the primarywinding and the secondary winding, to be injected into the secondarywinding. The primary shield is connected to the “quiet termination” ofthe primary which can be primary GND, 150 or Bulk+, 100. The shieldplaced on the layer 500 will short the displacement current induced bythe primary winding via the parasitic capacitance between the primarywindings and said primary shield, towards the primary “quiettermination”, such as Bulk+, 100 and Input GND 150. In this way saiddisplacement current is prevented from flowing towards the secondarywindings and further into the secondary “quiet termination”.

In the first embodiment of this invention is claiming a planartransformer formed by a magnetic core, 180, a primary multilayer PCB,220, which contains the primary winding and some auxiliary windingsreported in respect of safety to the primary but none of the secondarywindings or auxiliary windings reported in respect of safety to thesecondary, and a secondary multilayer PCB board 280 which contains atleast the secondary winding and none of the primary windings orauxiliary windings reported in respect of safety to primary side. Saidprimary board, 220 and said secondary board 280 are placed adjacent toeach other. The secondary board can be placed on the top of the primaryboard, or the primary board can be placed on top of the secondary board.The layer on the surface of said primary board, adjacent to thesecondary board contains the shield and said shield is connected to saidprimary “quiet termination”. In some application in order to furtherdecrease the parasitic capacitance between primary and secondary aspacer may be placed in between said primary and secondary boards. Thatmay increase the leakage inductance, though in some application theenergy contained in the leakage inductance may be harvested. Thecapability to control the parasitic capacitance between primary andsecondary it will be very useful in application wherein the common modeEMI has to be at a very low level, while maintaining a very low leakageinductance. Such application is the power converters in medicalapplications.

We define the shield winding as a winding which has a given shape and agiven polarity reported to the connection to the primary “quiettermination”. The shape of the shield winding will impact the parasiticcapacitance between primary shield and the secondary windings. The shapeof the shield winding also dictates the amplitude of the voltage acrossthe shield winding and its impedance in reference to the connection tothe “quiet termination”. A shield winding with several turns and partialturns have a higher voltage at the termination of the shield winding notconnected to the primary “quiet termination” increasing in this way thedisplacement current towards the secondary winding or towards thesecondary shield. A wider winding will have a larger parasiticcapacitance towards the secondary winding which is placed in theproximity of the primary shield. In conclusion the shape of the primaryshield controls the parasitic capacitance towards the secondary windingfrom the secondary board and also the amplitude and polarity of theinjection voltage which creates the displacement current between primary“quiet termination” and the secondary windings which are part of thesecondary board. In this way the common mode noise in between theprimary “quiet termination” and secondary “quiet termination” can befully controlled in amplitude and polarity by the design of the primaryshield placed on top layer of the primary board adjacent to thesecondary board. The primary shield has two functions, the firstfunction is to short the displacement current produced by the primarywindings via the parasitic capacitances between primary windings and theprimary shield, to the primary “quiet termination”. The second role ofthe primary shield is to create a displacement current in between theprimary “quiet termination” and the secondary winding from the secondaryboard, displacement current of a controlled amplitude and polarity insuch a way that the common mode noise in between the primary “quiettermination” and secondary “quiet termination” to be zero.

Second Embodiment

In FIG. 8 is presented the second embodiment of this invention. Theprimary board, 220 contains a shield placed in the top layer, 500. Theother layers, 530, 540, 550 and 560 are sandwiched in between the toplayer 500 and bottom layer 510. The winding contained on the layer 530adjacent to layer 500, is connected to the Vbulk+. For example if theprimary winding 110 has 12 layers, on each layer such as 530, 540, 550and 560, will contain 3 turns. The voltage swing on the windingcontained in layer 530, adjacent to the shield, 500, will have a lowervoltage swing towards the “quiet termination” Vbulk+ than the rest ofthe windings, and the winding contained in layer 560 will have thelargest voltage swing towards the “quiet termination” Vbulk+, and wewill refer to it in this invention as the “noisy” winding. The windingon layer 540, 550 will be also called “noisy” windings even if theamplitude of the voltage swing in reference to the primary “quiettermination” is smaller than the winding on layer 560. The shield placedon the layer 500 is connected to Vbulk+ in this example. The shieldplaced on the layer 500 can be also connected to the input GND, 150which is also a primary “quiet termination”. The goal is to reduce theamplitude of the displacement current injected in the shield placed onlayer 500, from the rest of the windings. For this purpose, we place the“quiet” windings on layer 530 which is adjacent to primary shield layer500. This placement will decrease the amplitude of the displacementcurrent in between the parasitic capacitance between layer 500 and layer530. The reduction of the displacement current will decrease theswitching losses in the primary windings. The secondary PCB, 280, isplaced on top of the primary PCB 220 as depicted in FIG. 8. For safetycompliance, the top layer and the bottom layer of the secondary boardthere will not have copper. In addition to that the distance between thewinding placed on layer 590 and top layer 240, distance, 610 and thedistance between layer 600 and layer 250 will have a safety approveddistance, 610, as it was depicted in FIG. 3. Function of the RMSvoltages the windings in primary and secondary that safety distance mayvary. In the second embodiment of this invention there will not be ashield in the secondary board. This will allow to use a secondary PCBwith a reduced number of layers such as 2 or 4 layers. In the casewherein 4 layer PCB is used for the secondary board, 280, we use all 4inner layers, 600, 580, 570 and 590 for secondary windings. On thesecondary board, 280, we can parallel some of the windings as depictedin FIG. 8. In FIG. 8 the windings on layer 600 are placed in parallelwith the winding from layer 580 and the windings on layer 570 is placedin parallel with the windings from layer 590. Because the currentsharing in the parallel winding it is not equal in asymmetricalstructure as the one from FIG. 8, because the secondary does notsandwich the primary windings, the current amplitude for the secondarywindings will be higher for the windings closer to the primary winding.To improve the current sharing the winding from layer 600 shall beparalleled with the winding from layer 590 and the winding from layer580 shall be in parallel with the winding from layer 570. A seriesconnection of the winding in secondary board will ensure a more equaldistribution of the current through the secondary layers.

The location of the rectifier means, 140 and the connection of thesecondary winding, which is connected to secondary GND, 160 at the dot,creates a configuration referred in the power conversion field as “highside rectifier”.

Without a shield in the secondary the voltage swing of the winding onlayer 600 will inject a displacement current into the shield 500 throughthe parasitic capacitance between the secondary winding in the layer 600and the shield 500 in the primary, displacement current which will flowinto the primary ground. The current flow from the secondary GND, 160towards the primary GND, 160 is referred in this patent as “common modenoise”. The voltage swing in the secondary winding on layer 600 versusthe shield 500 of the primary PCB, 220, is lower than the voltage swingon the winding placed in the other layers. To further minimize thiscommon mode noise, the shield 500 has to te have the exact imagine asthe winding from layer 600 and the same orientation as the secondarywinding from layer 600 and the winding from layer 500 will have avoltage swing of the same amplitude and with the same polarity. Jitaru'spatent application “Power Transformer for Minimum Noise Injection inbetween primary and secondary winding Rompower Active Shield”, U.S.patent application Ser. No. 16/732,240 filed on Dec. 31, 2019, depicts asimilar concept but with some clear differences.

In FIG. 7 is depicted a configuration for the secondary referred in thepower conversion field as the “low side rectifier” The winding containedin the layer 600 is connected to a “quiet termination” which is Vo+, 320in order to minimize the noise injecting from the secondary windingplace on the layer 600 into the primary shield, 500, from the primaryboard, 220.

Like in the high side configuration, to minimize this common mode noise,the shield 500 has to be have the exact imagine on the winding fromlayer 600 and the same orientation as the winding from layer 600 and thewinding from layer 500 will have a voltage swing of the same amplitudeand with the same polarity.

The second embodiment of this invention has all the claims of the firstembodiment and in addition to this the secondary board does not have alayer allocated for the shield and the layer of the secondary boardadjacent to the primary board, layer which contains the primary shield,is connected to the secondary “quiet termination” to Vo+, 320 for thelow side implementation in the secondary or to the “quiet termination”,secondary GND, 160 in the high side implementation of the secondary. Inaddition to that the primary shield, placed on the primary boardadjacent to the secondary board, shield connected to the primary quietspot, Vbulk+, is the mirror imagine and the same polarity of the windingon the layer connected to secondary “quiet termination”, such as thesecondary GND, 160, in the high side secondary configuration or to Vo+,320 in the low side secondary configuration.

Third Embodiment

In FIG. 9 is presented the third embodiment of this invention. Theprimary board, 220 contains a shield placed on the top layer, 500, andunderneath are placed the other transformer winding of the primaryboard. In the secondary board we will have also a shield placed on thelayer 600, a layer adjacent to the layer 500 of the primary board whenthe secondary board 280 is placed on top of the primary board 220. Thesecondary windings are placed on the layers 580 and 570. The layer 590can be used as a shield or for other purposes.

Besides the common mode noise caused by the displacement current throughthe parasitic capacitance between the primary winding and the secondarywinding, the common mode noise can be produced also by the noiseinjection between the magnetic core and the primary and secondarywinding or by other noise sources in the layout referred in this patentas “non transformer common mode noise”. For example a noise injectioncan be produced by the “noisy section” of the primary circuit connectedto the drain of the primary switch 130, which has a very high dv/dt,which will inject noise via the parasitic capacitances between said“noisy section” of the primary circuit and the secondary components.

In conclusion the total suppression of the common mode noise betweenprimary and secondary windings in the transformer does not totallyeliminate the common mode noise.

In order to further reduce the common mode noise in a power converter anoise cancelation technique should be employed. The noise cancellationtechnique consists in injecting a signal, between primary and secondaryground, signal which has the proper amplitude, polarity and phase whichwill cancel the noise injected via the means previously described.

One method of noise injection can be done between the primary shield andthe secondary shield. In the multilayer PCB magnetic technology asdescribed by Jitaru in U.S. Pat. No. 5,990,776, the parasiticcapacitances between primary and secondary winding can be bettercontrolled in production than the traditional wire wound magnetictechnologies.

A “dual function shield”, it is shield which does have two functions.One primary function of the “dual function primary shield” is to createa low impedance path for the noise injected by the transformer primarywindings, to the primary “quiet termination” and prevent that noise toreach the secondary windings and secondary shield, when the secondaryshield does exist, and to prevent reaching the secondary “quiettermination”. The second role of the primary “dual function shield” isto inject into the secondary windings, including the secondary shield,when secondary shield does exist, a displacement current of an amplitudeand polarity designed to cancel the common mode noise between primary tothe secondary of the power converter in a such way that the total commonmode noise in between primary and secondary to be significantly reducedtowards zero.

This definition does apply also to the dual function shield in thesecondary. A “dual function shield” in the secondary, it is shield whichhas two roles. One primary role of the “dual function secondary shield”is to create a low impedance path for the noise injected by thetransformer secondary windings towards the secondary “quiet termination”and prevent that noise to reach the primary windings and primary shield,when the primary shield does exist, and prevent further flow of saidnoise to the primary “quiet termination”. The second role of thesecondary “dual function shield” is to inject into the primary windings,including the primary shield, when the primary shield does exist, adisplacement current of an amplitude and polarity designed to cancel thecommon mode noise between secondary to the primary of the powerconverter in a such way that the total common mode noise in betweenprimary and secondary of the power converter to be significantly reducedtowards zero.

“Controlled dual function shield” it is a “dual function shield” whereinthe noise injection between primary and secondary is adjustable andcontrolled.

In FIG. 6A is presented a configuration for the “Controlled dualfunction shield” for the primary and secondary shield which can be usedto tailor the amplitude and polarity of the noise injection in order tominimize the common mode noise in a power converter. Initially we willdescribe the primary side “Controlled dual function shield”.

The primary “Controlled dual function shield” is formed by a conductivematerial placed on the primary shield layer, 500. The “controlled dualfunction shield” is connected to a primary “quiet termination”, 330,from FIG. 6A. In the design phase of the transformer the shield isformed by multitude of cells, 400, connected in between by a short trace430, for electrical connectivity. The voltage induced in each cell has agiven amplitude and a polarity, which changes during the converteroperation, as depicted by 370, and 380. The cells are partially cut outby slots such as 420 and 410 and connected in between by a short tracesuch as 430.

On this shield there is also a total cutout, 340 slot. The location ofthe total cutout slot, 340, versus the connection to the “quiettermination” plays a key role in noise injection.

As depicted in FIG. 6C, a base line, 720, unites the center of themagnetic core center post, 334, and the connection to the “quiettermination”, 330. The “cutout line”, 730, is defined by the center ofthe magnetic center post, 334 and the total cutout, 340. An angle “α”,333, represents the angle in between the main line and cutout line. Thecutout, 340, can shift clockwise, in the direction, 352, or contraclockwise, in direction, 360 as depicted in FIG. 6A. Shifting the totalcutout we do modify the displacement current between the primary shieldand secondary layer placed on the secondary board adjacent to theprimary board. For example, in FIG. 6D, the total cutout, 340, isdiametral oppose to the primary connection to “quiet termination” 330.In FIG. 6B and FIG. 6C are depicted another example of cutout placement,which will control the amplitude and polarity of the displacementcurrent between primary and secondary shield.

In the tunning process, the total cutout 340, starts from the cellsclose to the connection to the “quiet termination”, (α=0) and moveclockwise and contra clockwise and measure the common mode noise at theinput EMI filter. In this way by rotating the location of the cutout,340, the optimal location of the total cutout can be identified, whereinthe common mode noise has the lowest amplitude.

As depicted in FIG. 6D with the polarity depicted by 370, in the rightside of the total cutout, 907, reported to 330, there is a positive halfturn voltage amplitude while in the left side of the total cutout, 903,there is a negative half turn voltage amplitude. For a secondary board,280, placed on top of said primary board, 220, with a shield winding600, adjacent to said primary shield and with the bottom layer of saidsecondary board, 250, not metalized, and having said primary shield asdepicted in FIG. 6D, the displacement current produced by left side ofthe primary shield, 903, is cancelled by the displacement currentproduced by the right side of the primary shield, 907.

By rotating the total cutout, 340, clockwise or contra clockwise thenoise injection between the primary shield, 500 into the secondaryshield 600, can be controlled in amplitude and in polarity. This tunningcan be done at the level of prototype. In large production, because inmultilayer PCB magnetic technology the consistency of the parasiticcapacitance between windings is very good, the location of the totalcutout 340 will be kept after is optimized in the prototype. In thesecond embodiment of this invention, the primary winding has a shield onthe top later, said primary shield which is connected to the primaryGND. There is also a shield located on the bottom layer of the secondaryboard, above the non-metalized, layer, wherein the bottom layer of thesecondary board is placed above the top layer of the primary board.

A primary “controlled dual function shield” is formed by an electricallyconductive material placed on the primary shield layer and connected toa primary “quiet termination”. By moving the cutout, 340, clockwise orcontra clockwise, in reference to said “quiet termination” the polarityand amplitude of noise injected by said “controlled dual functionshield” to the secondary winding placed on the secondary board adjacentto the “controlled dual function shield”, the noise injection can becontrolled.

The same type of shield can be implemented for the secondary shield andthe connection 330 in the secondary shield will go the secondary “quietterminal” such as secondary ground, 160 or Vo+, 320 from FIG. 1. Saidtotal cutout, 340, of the primary shield and the total cutout of thesecondary shield are moved clockwise or counterclockwise until the noiseinjection between the primary shield and the secondary shield leads tototal cancelation of the common mode. Once the locations of the totalcutouts are identified, the position will be held fixed in theproduction setting. The displacement current in between de primaryshield and the secondary shield, is tailored to cancel the common modenoise in such a way that the total common mode between primary andsecondary is zero. In some application the noise cancelation amplitudehas to be higher and in such cases the shield winding in primary andsecondary can have the shape depicted in FIGS. 6E and 6F. The voltagesat the termination of the shield are Vx+ and Vx−. By tailoring the shapeof the primary and secondary shield and its polarity a controlleddisplacement current can be injected in between primary and secondary insuch way that the common mode current which flows in between saidprimary and secondary ground is zero.

In this embodiment we presented the “controlled dual function shield” inplanar transformers. The same concept can apply also in conventionaltransformers wherein the shield winding will have the shield effect andalso the controlled noise injection by tunning the number of turns inthe shield.

Fourth Embodiment

The primary board 220, can be part of the mother board containing mostof the electronic components. This solution has the advantage that iteliminates the electronic interconnection pins. This solution isdescribed in the U.S. Pat. No. 5,990,776. In the solution presented inthe U.S. Pat. No. 5,990,776 all the windings such as primary winding,the secondary windings and the auxiliary windings primary windings canbe buried inside of the multilayer PCB, referred also as the motherboard and some of the components and some power components can be placedon top of the transformer winding. This prior art solution from U.S.Pat. No. 5,990,776, has several drawbacks. The major drawback is thatthe number of layers required for the primary windings, the secondarywinding and the auxiliary windings will lead to an expensive motherboard because the large number of layers which is required. In thisembodiment the secondary board, 280, containing the secondary windingwill be a smaller PCBs which is placed above the mother board asdepicted in FIG. 11A. The interconnection between the secondary boardand the primary board can be done with simple, plated half hole cut asdepicted in FIG. 14. The plated half hole cut allows the solder tocreate a very good connection and the plate around the half hole cutensure a very good electrical connection. Other solutions such asinterconnection pins can be also used.

In some applications there are not layers available for a shield in thesecondary. In such cases the secondary winding which is connected to asecondary “quiet termination” which is placed on the bottom layer whichis metalized, such as layer 600 from FIG. 3. This case is depicted inFIG. 7, and FIG. 8. Because no shield is placed in the layer 600adjacent to the primary board, a displacement current is produced bysaid secondary winding placed on layer 600 into the primary shield 500,said displacement current which is produced by the voltage swing in thesecondary winding which is placed on layer 600. In such cases thedisplacement current produced by the secondary winding from layer 600into the primary shield 500, can be cancelled by the displacementcurrent injected by the primary shield 500 which is a controlled dualfunction shield employing the adjustable noise injection technique fromembodiment 3.

Fifth Embodiment

In this embodiment, the transformer is formed by a primary board 220,wherein the primary windings are embedded, and the secondary windingsare embedded into the secondary board which is soldered to the primaryboard as presented in FIG. 11A. The board 220 is also the mother boardwherein the components of the AC-DC converter are placed.

Additional components such as 810 from FIG. 11A are placed on thesecondary board. Such components can be the synchronized rectifiers,140, the controller of the synchronized rectifier and some of the outputcapacitors part of Co, 170. By placing the synchronized rectifier, 140and some of the Co, 170 the stray inductance in between the secondarywinding 120 and the Co, is reduced and “the end effects” associated withthe secondary wining, 120 are also decreased.

Sixth Embodiment

In U.S. Pat. No. 6,466,454 B1 are presented different methods ofinterconnecting multiple boards and using one magnetic core. Thispackaging concept wherein the primary windings are placed on the motherboard has additional advantages, such as the use of windings around theouter leg of the transformer for different applications such asauxiliary windings and fractional turns. In this embodiment there arenot interconnection pins neither for the primary winding board andneither for the secondary board. In some of the applications, the copperthickness for the secondary winding needs to be higher, leading to amore expensive board. Placing the secondary windings on a more expensiveboard but of much smaller dimensions, leads to a lower total cost forthe multilayer planar transformer comparative with more conventionalsolutions wherein the primary and the secondary windings are placed onthe same multilayer PCB. In this patent key embodiment wherein thesecondary board containing the secondary windings is placed on one side,top or bottom, of the multilayer PCB containing the primary winding willincrease the leakage inductance between the primary and secondarywinding. A solution is to place two secondary boards containing thesecondary winding, one secondary board on one side of the primary boardand another secondary board on the other side of the primary board asdepicted in FIG. 11B.

In applications wherein the power density it is very high and we cannotafford to lose space for the components due to the magnetic core, theentire transformer can be elevated and there will be two boards, oneprimary board 220 which is connected to the mother board via theconnectors, 460, and a secondary board, 280, connected to the motherboard by connectors, such as 450. In FIGS. 12 and 13 is depicted the twoboards planar transformer with the primary board on the bottom and thesecondary board on the top and vice versa.

Seventh Embodiment

Another drawback of the previous embodiments is that the transformerbuilt by a primary PCB and the secondary PCB placed at one side of theprimary board increases the leakage inductance in between the primaryand the secondary wining.

In the U.S. patent application Ser. No. 14/660,901, Magnetic StructuresFor Low Leakage Inductance And Very High Efficiency—U.S. patentapplication Ser. No. 14/660,901 filed Mar. 17, 2015 and further inMagnetic Structures For Low Leakage Inductance And Very HighEfficiency—U.S. patent application Ser. No. 14/660,901 filed Mar. 17,2015, is described a magnetic structure using multiple legs wherein theprimary windings encircles successively the multiple posts, and also thesecondary winding encircles successively the multiple posts. In such astructure the number of layers necessary for the primary windings arereduced.

In such magnetic structure the leakage inductance in between primary andsecondary is reduced and that helps lowering the leakage inductancebetween the primary and secondary windings even if we place thesecondary winding on a separate board at one side of the primary PCB.

In FIG. 15 is depicted such a structure. There is a primary board 220,and a secondary board on top of it, 280. A multiple posts core 410, withposts 420, are placed in such way that the posts, penetrate through theholes 430 placed in the secondary board, 280 and the primary board 220.A magnetic plate 415 is placed on the other side of said posts 420 toclose the magnetic field path.

Eighth Embodiment

In FIG. 16 is depicted another multiple board structure in building themagnetic for a two-transistor forward topology. This topology containstwo switching elements, M1, 305 and M2, 315 which are placed in serieswith the primary winding. Two reset diodes Dr1, and Dr2 are also part ofthe power train. In this topology both switchers M1 and M2 do turn onand off at the same time. As a result the middle of the primary windinghas zero or a negligible voltage swing reported to the “quiettermination” such as V_(Bulk)+, 100. In such application the secondarywindings shall be placed in the middle of the primary windings asdepicted in FIG. 16. The primary winding is placed on the primary board220, which can be also the primary board and the secondary windings aredistributed in such way that the windings on the top and bottom surfaceof the primary board are the windings in the middle of the primarywindings such as 440 and 445. The secondary windings are part of theboard 280 and 282. These boards are placed on top and the bottom ofprimary board 220.

In the drawing 16, the secondary boards, 280 and 282 do have a layerallocated for the secondary shield, layers which are adjacent to theprimary board. There is no shield in the primary due to a very lowvoltage swing in the winding 440 and 445. However, in some cases whereinthe EMI level must be very low two primary shields can be placed nearlayer 440 adjacent to the secondary board 280 and another shield can beplaced in the primary board 220 close near the winding 445 on the layeradjacent to the secondary board 282. These two primary shields should beconnected to the primary “quiet termination” such as Vbulk+, 100 orprimary ground, 150.

Ninth Embodiment

In FIG. 17 is depicted another multiple board structure in building thetransformer for a flyback topology. In some flyback applications theleakage inductance energy is harvested and used for other purposes.However, at higher power flyback the leakage inductance must be kept lowin order to increase the efficiency because the harvested energy will betoo high. In FIG. 17 is presented such a case wherein the leakageinductance of the magnetic structure using multiple multilayer PCB iskept low by the use of an interleaving transformer structure wherein theprimary boards 220 and 222 are placed on top and the bottom of thesecondary board 220. The winding for the primary starts on the primaryboard 222 on the layer 730 and continue to the layer 740 and 750 fromthe primary board 222. Further the primary windings go the primary board220 to the layers 910, 920 and 930 and the connection goes further tothe drain of primary switch 130, In the primary there are placed twoshields 700, on the primary board 222 and 900 on the primary board 220.In the drawing depicted in FIG. 17, there are not shield in thesecondary winding 280. Is recommended that the primary shields 900 and700 to be “Controlled double function shield” because the shield 900 isplaced in vicinity of “noisy” windings such as the winding on layer 910.In addition to that the secondary board 290 does not have a shield. Inthe event shields will be placed on the secondary board, 280,“controlled double function shield” recommended. In such case the“controlled double function shield” in primary and secondary can betuned in such way that the total common mode noise of the converter canbe reduced to zero. In this embodiment, the secondary board is placed inthe middle of the primary winding wherein the noise level is high,unlike the two-transistor forward topology. By employing two shields inthe primary boards, 220 and 222 and of the type “Controlled doublefunction shield” the total common mode in the power converter can bereduced substantially and if the secondary would employ the same“Controlled double function shield” the common mode noise of the powerconverter can be easily reduced to zero in spite of the fact that thesecondary board is placed in between two primary boards wherein thewindings are very noisy.

Preferred embodiments are fully and clearly described above so as toenable one having skill in the art to understand, make, and use thesame. Those skilled in the art will recognize that modifications may bemade to the description above without departing from the spirit of thespecification, and that some embodiments include only those elements andfeatures described, or a subset thereof. To the extent thatmodifications do not depart from the spirit of the specification, theyare intended to be included within the scope thereof.

The invention claimed is:
 1. A system for reducing common-mode noise,the system comprising components of: a switch mode power supplyincluding primary and secondary sides, primary and secondary sidegrounds, an input voltage source, a primary switch, a transformer, acore, and a power output, wherein the primary side and the secondaryside have primary and secondary quiet terminations, respectively,wherein voltage does not change with respect to the primary side groundand with respect to the secondary side ground; a magnetic elementcontaining a magnetic core having at least two magnetic legs; a primarymultilayer board which embeds primary windings and first auxiliarywindings which report to the primary windings, wherein the primarywindings and the first auxiliary windings surround the magnetic legs; asecondary multilayer board which embeds secondary windings and secondauxiliary windings which report to the secondary windings, wherein thesecondary windings and second auxiliary windings surround the magneticlegs; wherein the magnetic core penetrates both the primary multilayerboard and the secondary multilayer board; wherein the primary andsecondary multilayer boards are placed on top of each other; and a layerof the primary multilayer board which is adjacent to the secondarymultilayer board includes a controlled dual function shield which isconnected to the primary quiet termination.
 2. The system for reducingcommon-mode noise of claim 1, wherein some of the components of thesystem are placed on the primary multilayer board.
 3. The system forreducing common-mode noise of claim 1, wherein some of the components ofthe system are placed on the primary multilayer board and on thesecondary multilayer board.
 4. A system for reducing common-mode noise,the system comprising components of: a switch mode power supplyincluding primary and secondary sides, primary and secondary sidegrounds, an input voltage source, a primary switch, a transformer, acore, and a power output, wherein the primary side and the secondaryside primary and secondary quiet termination, respectively, whereinvoltage does not change with respect to the primary side ground and withrespect to the secondary side ground; a magnetic element containing amagnetic core having at least two magnetic legs; a primary multilayerboard which embeds primary windings and first auxiliary windings, whichreport to the primary windings, wherein the primary windings and thefirst auxiliary windings surround the magnetic legs; a secondarymultilayer board which embeds secondary windings and second auxiliarywindings which report to the secondary windings, wherein the secondarywindings and the second auxiliary windings surround the magnetic legs;wherein the magnetic core penetrates both the primary multilayer boardand the secondary multilayer board; wherein the primary and secondarymultilayer boards are placed on top of each other; a layer of thesecondary multilayer board which is adjacent to the primary multilayerboard includes a controlled dual function shield which is connected tothe secondary quiet termination.
 5. The system for reducing common-modenoise of claim 4, wherein some of the components of the system areplaced on the primary multilayer board.
 6. The system for reducingcommon-mode noise of claim 4, wherein some of the components of thesystem are placed on the primary multilayer board and on the secondarymultilayer board.
 7. A system for reducing common-mode noise, the systemcomprising components of: a switch mode power supply including primaryand secondary sides, primary and secondary side grounds, an inputvoltage source, a primary switch, a transformer, a core, and a poweroutput, wherein the primary side and the secondary side have primary andsecondary quiet terminations, respectively, wherein voltage does notchange with respect to the primary side ground and with respect to thesecondary side ground; a magnetic element containing a magnetic corehaving at least two magnetic legs; a primary multilayer board whichembeds primary windings and first auxiliary windings which report to theprimary windings, wherein the primary windings and the first auxiliarywindings surround the magnetic legs; a secondary multilayer board whichembeds secondary windings and second auxiliary windings which report tothe secondary windings, wherein the secondary windings and the secondauxiliary windings surround the magnetic legs; wherein said magneticcore penetrates both the primary multilayer board and the secondarymultilayer board; wherein said primary and secondary multilayer boardsare placed on top of each other; a layer of the secondary multilayerboard which is adjacent the primary multilayer board includes acontrolled dual function shield which is connected to the secondaryquiet termination; and a layer of the primary multilayer board which isadjacent to the secondary multilayer board includes a controlled dualfunction shield which is connected to the primary quiet termination. 8.The system for reducing common-mode noise of claim 7, wherein some ofthe components of the system are placed on the primary multilayer board.9. The system for reducing common-mode noise of claim 7, wherein some ofthe components of the system are placed on the primary multilayer boardand on the secondary multilayer board.
 10. A system for reducingcommon-mode noise, the system comprising components of: a switch modepower supply including primary and secondary sides, primary andsecondary side grounds, an input voltage source, a primary switch, atransformer, a core, and a power output, wherein the primary side andthe secondary side have primary and secondary quiet terminations,respectively, wherein voltage does not change with respect to theprimary side ground and with respect to the secondary side ground; amagnetic element containing a magnetic core having at least two magneticlegs; a primary multilayer board which embeds primary windings and firstauxiliary windings which report to the primary windings, wherein theprimary windings and the first auxiliary windings surround the magneticlegs; two secondary multilayer boards which embed secondary windings andsecond auxiliary winding which report to the secondary windings, whereinthe secondary windings and the second auxiliary windings surround themagnetic legs; the secondary multilayer boards are on sides of theprimary multilayer board; wherein the magnetic core penetrates theprimary multilayer board and both secondary multilayer boards; and twolayers of the primary multilayer board which are adjacent to thesecondary multilayer boards include a controlled dual function shieldwhich is connected to the primary quiet termination.
 11. The system forreducing common-mode noise of claim 10, wherein some of the componentsof the system are placed on the primary multilayer board.
 12. The systemfor reducing common-mode noise of claim 10, wherein some of thecomponents of the system are placed on the primary multilayer board andon the secondary multilayer board.
 13. The system for reducingcommon-mode noise of claim 10, further comprising layers of thesecondary multilayer boards which are adjacent to the primary multilayerboard and which include a controlled dual function shield that isconnected to the secondary quiet termination.
 14. The system forreducing common-mode noise of claim 13, wherein some of the componentsof the system are placed on the primary multilayer board.
 15. The systemfor reducing common-mode noise of claim 13, wherein some of thecomponents of the system are placed on the primary multilayer board andon the secondary multilayer board.
 16. A system for reducing common-modenoise, the system comprising components of: a switch mode power supplyincluding primary and secondary sides, primary and secondary sidegrounds, an input voltage source, a primary switch, a transformer, acore, and a power output, wherein the primary side and the secondaryside primary and secondary quiet terminations, respectively, whereinvoltage does not change with respect to the primary side ground and withrespect to the secondary side ground; a magnetic element containing amagnetic core having at least two magnetic legs; a secondary multilayerboard which embeds secondary windings and secondary auxiliary windingswhich repot the secondary windings, wherein the primary windings and thesecondary auxiliary windings surround the magnetic legs; two primarymultilayer boards which embed primary windings and first auxiliarywindings which report to the primary windings, wherein the primarywindings and first auxiliary windings surround the magnetic legs; theprimary boards are on sides of the secondary multilayer board; whereinthe magnetic core penetrates the secondary multilayer board and bothprimary multilayer boards; and two layers of the secondary multilayerboard which are adjacent to the primary multilayer boards include acontrolled dual function shield which is connected to the secondaryquiet termination.
 17. The system for reducing common-mode noise ofclaim 16, wherein components are placed on the primary multilayer board.18. The system for reducing common-mode noise of claim 16, wherein someof the components of the system are placed on the primary multilayerboard and on the secondary multilayer board.
 19. The system for reducingcommon-mode noise of claim 16, further comprising layers of the primarymultilayer boards which are adjacent to the secondary multilayer boardand which include a controlled dual function shield that is connected tothe primary quiet termination.
 20. The system for reducing common-modenoise of claim 19, wherein some of the components of the system areplaced on the primary multilayer board.
 21. The system for reducingcommon-mode noise of claim 19, wherein some of the components of thesystem are placed on the primary multilayer board and on the secondarymultilayer board.